that is rather hard to answer actually considering that you only have a single life to live, but if your monster is starting to flag you either rebuild early or you go and get yourself a new one and sell it off to someone who is willing to do the work. Or it simply gets involved in an accident and is beyond repair.
Cars are different in that way: most people will drive them into the ground because they could sell them for more as parts perhaps and an improperly functioning car will not get you killed nearly as quickly as an improperly functioning motorcycle.

My engine has a "history" and died at 45.000 km. There's high wear on the valve train and the main bearings crashed. Damage (repaired) from a broken engine bolt and a cracked crankcase early in life may have contributed to the short lifespan. Infrequent oil changes didn't help either...

A friend of mine got 70,000 miles out of his 94 900ss. I could smell (but not see) a faint oil scent when riding down hill behind him and it still seemed to have plently of life left in it when he threw it away on Palomar Mtn.* *